Friday, 14 September 2007

Lapping sucks

This week has certainly not included any of my childish reward systems for good behavior. I lapped a 16-1/2” African Blackwood panel plane on Monday. Did some other work on Tuesday and wednesday to give my abs of sorrow a break, but then returned to lapping madness on Thursday to lap a 14-1/2” Rosewood panel plane. And guess what… I lapped another A1 today. Say hello to abs of jello.

Lapping the sole and the sides of a plane is pretty sweaty, filthy work.. but a necessary evil if you are a planemaker. Yeah – I know… why don’t you buy a surface grinder? That will be the subject of another discussion.

When Joe and I started making planes, we tried every type and manufacturer of sandpaper we could find. Every paper we tried was terrible – the grit lasted about 2 minutes, the paper curled, tore and was messy. Then we found “Champaign Magnum” – a curious name for our hero paper. This is automotive sandpaper made by Norton and currently goes under the more manly name A275. It is a cloth backed paper, so it does not curl like paper backed sandpaper. It has some sort of coating that keeps it from loading but does not interfere with the cutting action of the grit. I buy this stuff by the 50 sheet box at a local Carquest. It is very reasonably priced – about $.50 a sheet. I typically go through 12 sheets on a panel plane - lapping both sides and the sole. A steel sided plane may take 16 sheets as the 01 tool steel is quite a bit harder than the bronze sides.

Lapping the sole of a plane is a little like honing the back of a plane iron – it will only be as flat as the substrate you are working on. In my case, I have 3 lapping stations and one checking station. Two of the lapping stations are 1-1/2” thick, 12” wide, 48” long pieces of machine ground marble. I have been using these for years and they have worked extremely well. The two pieces of marble are built into the tops of two cabinets. I use a spray adhesive from 3M called “Super 77” to lightly dust the back of the paper and then stick it down to the marble. Be sure to wear a mask when you are using any sort of spray adhesive – you don’t want that stuff in your lungs. I place 4, 80 grit sheets end to end, being careful to remove any grit or crud from the marble beforehand.

The process of lapping is pretty simple looking – but there are several important things to keep in mind. Keep the blade in the plane, and held in place as if it were being used. This will ensure that any distortion caused by the blade holding mechanism will be taken into account while lapping. When I am lapping, I do not hold the plane as if I were using it, but rather hold it parallel to my body. I move side to side, being careful not to tip the plane forward or towards myself. Count the number of strokes you take on the fresh paper and take the same number of strokes with the plane rotated 180 degrees on another section of fresh paper. This is really important – you want to wear the paper as evenly as possible – just like using a waterstone. Be sure to let the toe and heel of the plane extend beyond the paper – this will help insure you are not just wearing the middle of the paper surface. If you are lapping steel, use a magnet to remove the dust. If you are using bronze or brass – use a shop vac with a good filter. Once the paper is worn out, peel it off and re-apply.

Check the surfaces for flat and square often. And here is where time and practice factors in. There are times when you need to deliberately remove more material on one edge to bring it to square. I am not quite sure how to explain this… but you have to “feel” where the paper is cutting and adjust your hand position and body pressure to suit. For anyone who sharpens without the aid of a jig and puts a camber on their iron – you know what I am talking about. If anyone has a more universal analogy – please add a comment at the end of this entry. It may also help to visualize where you want to remove material. Mark a grid on the sole of the plane - with lines spaced 1/2" apart. Also mark the high side that you need to remove material from. Now take two passes having adjusted your pressure and check the sole. If the grid is gone on the high side you are on the right track. If not - try again with a little more pressure on the high side. Over time, you will develop a "feel" for it... trust me.

My third lapping area is a fairly recent addition to my shop. It is a 4” thick, 12” x 36” granite reference surface. The literature says it is has a bilateral accuracy of plus or minus .0002”. I use 3 sheets of 180 grit and do the final lapping on this surface.

The last step is to check for flat – and this is done on another 4” thick, 12” x 36” granite reference surface. I place the plane on the surface and use a .001” feeler gauge and try to slip it under the plane all along the perimeter. On Thursdays panel plane (and the one in these photos), there was one corner that I could get the tip of the feeler under. For this stage of the process – I will call this flat knowing I have another crack at it when I file the mouth.

5 Comments:

The tape on the wood protects the finish. At this stage, there are about 12 coats of French polish on the wood and the last thing I need is a stray bit of steel or bronze scratching it up. That and my sweaty, fithy hands from lapping:)